1. Field of Invention
The invention relates to a device for supplying foil to a printing press, the foil being intended to be used in a printing process during which transfer portions of a layer of the foil are transferred to a substrate to be printed, the device comprising a foil input unit for inputting at least one foil web to the device, and a foil reuse unit for receiving a used length of foil web from the printing press and feeding the same length of foil web back to the printing press in a longitudinally shifted position so as to allow the printing press to take transfer portions from the length of foil web at positions of the length of foil web that have not been addressed during previous use of the length of foil web in the printing process.
Furthermore, the invention relates to a method for determining characteristics of control of a device for supplying foil to a printing press and a computer program product comprising a program code of a computer program for use in such a method.
2. Background and Related Art
In general, the invention is applicable in the field of applying a thin layer of a metal such as aluminum to a substrate such as paper or carton during a printing process, as in this field, there is a need for useful methods relating to supplying a foil comprising the thin metal layer and a carrier layer to a printing press. In this respect, techniques for transferring the metal layer from the carrier layer of the foil to the substrate have been developed that do not require any heating process for letting the desired transfer take place. The foil that is used in carrying out these techniques usually comprises a carrier layer made of polyethylene, wherein a layer of aluminum is deposited on this carrier layer. In the following, for sake of clarity, the term “transfer layer” will be used as a general term for the layer that is suitable for transfer to the substrate.
By using the foil as described in the foregoing, it is possible to realize printed matter having shiny, silver-like or gold-like portions, wherein the exact color of the portions is dependent on the color of the transfer layer of the foil that has been used during the printing process. In particular, the shiny portions are obtained by performing the following steps: applying a specific type of glue to predetermined areas on the substrate to be printed; and pressing the substrate and a length of the foil against each other, with the transfer layer of the foil facing the substrate. During the latter step, a transfer of the transfer layer from the carrier layer of the foil to the substrate to be printed takes place at the areas where the glue is present, so that the material of the transfer layer is arranged on the substrate according to the pattern of the glue.
The foil is often taken as a web from a coil that is unwound during a printing process in which the foil is used, so that the supply of foil can be continuous. In the printing press, the foil web is moved through a narrow gap between two cylinders of the printing press, wherein the substrate to be printed is arranged on one of the cylinders. At a position where the foil web is pressed against the substrate on the basis of pressing forces that are exerted on the foil due to interaction of the two cylinders of the printing press, predetermined portions of the transfer layer are transferred from the foil web to the substrate. After a length of foil web has passed the narrow gap between the cylinders of the printing press, and some portions of the transfer layer of the length of foil web have been used in the process of printing a substrate, the length of remaining foil is disposed of. For example, the foil web moves on to a device that is adapted to shred the foil.
A device as defined in the opening paragraph is known, for example, from EP 2 340 936 A1, which is hereby incorporated by reference. This patent publication relates to a method and a device for supplying foil to a printing press, which are suitable to be applied for reusing a length of foil web that has already been used in a printing process. In a practical embodiment of the device, a foil reuse unit is present, which is adapted to receive a used length of foil web from the printing press, to change a mutual position of the length of foil web and the printing press, and to feed the length of foil web back to the printing press. By changing the mutual position as mentioned, it is ensured that portions of a substrate to that transfer portions of a transfer layer of the foil need to be transferred are covered by unused portions of the length of foil web. The actions performed by the foil reuse unit can be repeated until there are no portions left on the length of foil web that can be used for covering portions of the substrate that need to be provided with the transfer portions. In general, reuse of a length of foil web, which may take place one time or more than one time, leads to reduction of waste of foil.
EP 2 340 936 A1 discloses an embodiment of the device in which it is not only possible to use a length of foil web more than one time, but also to divide a basic foil web in at least two smaller foil webs. In a practical case, one basic foil web may be divided in at least two smaller foil webs by performing a cutting process on the basic foil web. In this way, it is possible to create foil webs having a width that is smaller than a width of the substrate to be printed, wherein the width of the foil webs can be adjusted to the dimensions of the transfer portions of the transfer layer of the foil. In general, use of a number of smaller foil webs instead of a wider basic foil web leads to reduction of waste of foil.
It follows from the foregoing that EP 2 340 936 A1 discloses two different options of realizing optimal use of foil, namely by reusing lengths of foil web and by splitting a foil web. The two different options as mentioned may be combined, which is to say, a basic foil web can be divided in at least two smaller foil webs, and lengths of at least one of those smaller foil webs can be reused. It is an object of the invention to find a practical way of taking those two options into account and to further reduce waste of foil by enabling appropriate control of a device for supplying foil to a printing press.